Reduction of O2 to superoxide anion (O2•-) in water by heteropolytungstate cluster-anions

Fundamental information concerning the mechanism of electron transfer from reduced heteropolytungstates (POMred) to O-2, and the effect of donor-ion charge on reduction of O-2 to superoxide anion (O-2(center dot-)), is obtained using an isostructural series of 1e(-)-reduced donors: alpha-Xn+W12O40(9-n)-, Xn+ = Al3+, Si4+, P5+. For all three, a single rate expression is observed: -d[POMred]/dt = 2k(12)[POMred][O-2], where k(12) is for the rate-limiting electron transfer from POMred to O-2. At pH 2 (175 mM ionic strength), k(12) increases from 1.4 +/- 0.2 to 8.5 +/- 1 to 24 +/- 2 M(-1)s(-1) as Xn+ is varied from P5+ (3(red)) to Si4+ (2(red)) to Al3+ (1(red)). Variable-pH data (for 1(red)) and solvent-kinetic isotope (KIE = k(H)/k(D)) data (all three ions) indicate that protonated superoxide (HO2 center dot) is formed in two steps-selectron transfer, followed by proton transfer (ET-PT mechanism)-rather than via simultaneous proton-coupled electron transfer (PCET). Support for an outersphere mechanism is provided by agreement between experimental k(12) values and those calculated using the Marcus cross relation. Further evidence is provided by the small variation in k(12) observed when Xn+ is changed from P5+ to Si4+ to Al3+, and the driving force for formation of O-2(center dot-) (aq), which increases as cluster-anion charge becomes more negative, increases by nearly +0.4 V (a decrease of > 9 kcal mol(-1) in Delta G degrees). The weak dependence of k(12) on POM reduction potentials reflects the outersphere ET-PT mechanism: as the anions become more negatively charged, the "successor-complex" ion pairs are subject to larger anion-anion repulsions, in the order [(3(ox)(3-))(O-2(center dot-))](4-) < [(2(ox)(4-))(O-2(center dot-))](5-) < [(1(ox)(5-))(O-2(center dot-))](6-). This reveals an inherent limitation to the use of heteropolytungstate charge and reduction potential to control rates of electron transfer to O-2 under turnover conditions in catalysis.